There is an increasing need to monitor environmental parameters to a high degree of accuracy using cost-effective sensors that can be integrated into the structures to monitor a range of physical parameters. Applications of these sensors include monitoring of the fabrication process of advanced composites, non-destructive testing, vibration monitoring, crack detection in load-bearing members and others. The ability to perform structural health monitoring on a continuous real-time basis is fast becoming an important design aspect in the building of new intelligent civil structures such as dams, buildings, highways, offshore platforms and other engineering structures.
Deterioration of existing structures due to normal usage and damage resulting from natural disasters such as earthquakes can lead to serious loss of structural integrity. The presence and propagation of cracks and other structural flaws frequently go undetected and can develop to a full-scale catastrophic failure leading to unnecessary loss of life and property. In addition, without the benefit of a structural health monitoring system, the structural soundness of surviving buildings is unknown except through costly and time consuming manual inspections. Frequently, these inspections are highly laborious leading inadvertently in patchy and unreliable results. The potential of structural-integrated sensors to monitor a variety of structural health indicators can assist in providing reliable and objective results leading to an informed decision by the relevant building authorities.
In response to the increasing need for structural health monitoring, various methods are being developed and some of the most promising are based on the use of optical fiber sensors.
Fiber optic sensors may be categorized according to a number of classification schemes. Based on one scheme, fiber optic sensors may be classified as intrinsic if the effect of the measurand on the light being transmitted takes place in the fiber. The sensor is classified as extrinsic if the fiber carries the light from the source and to the detector but the modulation of light occurs outside the fiber. Another classification scheme divides the fiber sensors into how the optical properties are modulated in response to the physical perturbation to be monitored. Based on this method of classification, optical fiber sensors can be divided into intensiometric, polarimetric, interferometric and wavelength-based monitoring schemes.
In the domain of intensity-based systems, a number of sensor designs have been proposed. Their attractiveness lies in their relative ease of signal interrogation, which involves monitoring the light intensity level as opposed to phase-shifts or wavelength shifts, propagated through the optical fiber. The loss of optical signal intensity occurs around the sensitized region of the optical fiber. In general, the extent of the loss of optical intensity is related to the magnitude of the external perturbation. In some intensity-based optical design, a bi-stable system approach is adopted where the loss of intensity simply indicates the occurrence of an event, which is being monitored. These intensity-based systems can, in general, offer simplicity, versatility and reliability in applications where precise signal intensity measurement is not critical or required. Intensity-based sensors can be categorized as either fracture-based or strain-based sensor. Fracture-based sensors rely on the damage or fracture of the optical fiber itself e.g. in the event of impact or overload as a direct indication of host damage and are essentially a single-use system. Strain-based sensors, on the other hand, offer the possibility of continuous strain measurement in either static or/and dynamic loading. In this approach, damage is often inferred i.e. indirectly and this is achieved by observing and analyzing the strain response of the sensor for a given loading.
To date, only glass-based optical fibers have been employed for structural health monitoring. For civil engineering applications, particular when it becomes desirable to embed optical sensors within concrete structures, the extremely alkaline (pH 12) environment is known to be corrosive to standard glass fiber. In addition, the presence of moisture can weaken the glass core and accelerates crack growth in the fiber. Protection of the glass fiber by means of a polymer coating is required to help prevent the damage of the glass fibers due to the corrosive environment. In addition, glass-based optical fiber sensors are fragile—in general not amenable to handling and are highly susceptible to fracture.
It has been proposed that the glass optical fiber can be made sensitive to the measurand of interest (e.g. strain) without altering the physical makeup of the glass optical fiber. A precision-bored capillary tube manually drawn by heating a glass tube was used to house two cleaved multi-mode glass fibers. The application of strain causes a separation of the cleaved optical fiber end-faces by the air-gap resulting in the modulation of light intensity. Although the sensitivity of the sensor may be increased by using a longer capillary tube, this is done at the penalty of increased sensor size. The longitudinal separation between the two cleaved surfaces due to the air gap offers limited sensitivity to the externally applied strain since air can be assumed to be optically transparent within the strain domain of interest. In addition, the use of a precision bore glass tube is laborious adding to the manufacturing cost of the sensor. The susceptibility of the glass tube and glass fiber to fracture also reduces the attractiveness of the proposed design.
At least preferred embodiments of the present invention seek to provide an alternative optical fiber-based sensor design, which may address one or more of the above-mentioned drawbacks of existing designs.